During maternal fasting, glucose uptake by the ovine fetus decreases by more then 50% while the metabolic rate (oxygen consumption) does not change. In these circumstances, fetal catabolism may ensue with a diminished rate of fetal growth, and/or the fetus must rely on alternate fuels for continued survival. As amino acids may reasonably constitute a major source of alternate fuel, the proposed investigation will characterize the changes in amino acid utilization in the ovine fetus during fasting. Utilizing the chronic fetal sheep preparation in conjunction with in vitro tissue studies the following specific questions will be addressed. 1. Does the fetus receive an increased supply of individual or total amino acids from the placenta during maternal fasting? Quantitative fetal uptakes of glucose, oxygen and 25 amino acids will be determined in the same animals in the fed and fasted states by measuring venoarterial concentration differences and umbilical blood flow. 2. Does the fetus catabolize its own protein stores to derive additional substrate during maternal fasting? Hind limb venoarterial concentration differences of 25 amino acids, as well as muscle composition studies (amino acids, protein, DNA) will be performed in the fed and fasted states. 3. Is there evidence that the increased utilization of amino acids by the fetus during fasting reflects active gluconeogenesis? Liver tissue will be obtained from the same animals during the fed and fasted states. Key gluconeogenic enzymes, as well as fetal hepatocyte glucose production rates from gluconeogenic precursors, will be measured. In summary, the proposed research will characterize the manner in which the fetus is able to alter its amino acid metabolism during a period of decreased exogenous glucose supply.